The design of the trailing edge of an airfoil is preferably dictated by aerodynamic considerations. For improved aerodynamic performance, it is commonly preferred to provide a thin trailing edge for a gas turbine airfoil. However, thinness may result in weakness, and there are often structural limitations that limit the trailing edge design and necessitate the use of an aerodynamic design that is less than optimal.
It is known to use ceramic matrix composite (CMC) materials for airfoils and other components of gas turbine engines. CMC materials advantageously provide higher temperature capability than metal and a high strength to weight ratio. However, modern gas turbine engines have operating temperatures that may exceed even the high temperature limits of known oxide and non-oxide ceramic materials. Accordingly, a layer of insulating material may be used, which further exacerbates the trailing edge thickness issue, and/or active cooling channels may be provided, which further exacerbates the strength issue.
FIG. 1 illustrates a known arrangement for an airfoil 10 fabricated with a ceramic matrix composite material. FIG. 1 is a partial sectional view of the trailing edge portion 12 of airfoil 10. An outer shell of ceramic insulating material 14, such as the material described in co-owned U.S. Pat. No. 6,013,592, defines the airfoil shape. Respective suction side and pressure side layers 16, 18 of ceramic matrix composite material provide mechanical strength for the airfoil 10. The plies of reinforcing fibers (not shown) within each of these respective layers 16, 18 extend to the very end of the trailing edge 12 and are separate from each other. No ply is wrapped continuously around from the suction side 16 to the pressure side 18, because to do so would undesirably increase the thickness T of the trailing edge due to the minimum bend radius required for the material plies. A prefabricated CMC insert 20 is positioned between the suction and pressure side layers 16, 18 in order to define cooling channels 22. Pressure from the cooling air within channels 22 results in interlaminar stresses within the CMC layers 16, 18, which is the weakest direction of such a material. In addition, stress concentrations arise from the cooling channels themselves. Increasing the thickness of the CMC layers 16, 18 to add more strength results in an increase thickness T and it further exacerbates the cooling problem, since CMC materials have a relatively low coefficient of thermal conductivity.
FIG. 2 illustrates another known arrangement for an airfoil 24 fabricated with a ceramic matrix composite material. Airfoil 24 is illustrated with an outer shell of ceramic insulating material 25, but one skilled in the art may appreciate that such a device may be used with or without such an outer protective shell. In this arrangement, the plies of CMC material 26 extend continuously around the trailing edge portion 28 of the airfoil 24 from the suction side to the pressure side. This arrangement provides increased strength against interlaminar shear stresses. To achieve a desired outer surface profile with a desirably thin trailing edge thickness, geometry dictates that the plies separate along the centerline of the trailing edge included angle if both the inner and outer plies are bent to equivalent radii. Such shape results in the creation of void spaces 29 between adjacent plies of the CMC material 26. These void spaces 29 are only partially filled with matrix material in any of several known CMC matrix processes. For example, when the reinforcing fibers of the CMC material 26 are infused with a matrix material during a known chemical vapor infiltration (CVI) process, the exposed surfaces are preferentially coated, leaving voids where the fiber surfaces are separated. Alternately, during another known process of slurry-impregnated fabric lay-up, such as used in oxide-based CMCs, the slurry may not completely fill the void spaces 29 between plies 26 in this region. Furthermore, the slurry-based matrix undergoes extensive volumetric shrinkage during drying and firing, which will leave behind voids and/or cracks in the matrix-rich regions. As a result, the strength of the trailing edge portion 28 of airfoil 24 may be compromised. Furthermore, the fibers in the trailing edge region 28 between inner and outer plies are relatively unconstrained, resulting in poor control of fibers, uneven distribution of porosity, and variable properties.